On-line process analysis of industrial streams with flow-through cells containing optical sensors are known. Generally, for industrial process measurements, the sample stream is returned to the process line after the measurement in the flow cell containing an optical sensor so as to avoid product waste. Pretreatment of the process sample is often required prior to measurement.
Generally, a flow cell has two optical windows situated to permit electromagnetic radiation to pass through the first window, through the industrial process stream, and out through the second window. Fiber optic cables are commonly used to guide the input source radiation to the flow cell and also to guide the resulting partially “absorbed” light back to a remotely located spectrometer for measurement.
Simple flow cells constructed of two parallel optical plates are known. The distance between the optical plates is defined as the optical path length. The process liquid flow is analyzed as it passes through the space between the optical windows. Such flow cells have been used for on-line analysis of biological liquids, colloidal dispersed systems, and hydrocarbons. These flow cells are characterized by simple construction and can only be used for sample analyses that do not require sample preparation or pretreatment. Some analyses require careful temperature control, in these cases the flow cell may be placed in a thermostatically controlled bath. This may be particularly useful for reactions where the liberation of heat in the flow cell requires careful temperature control. These measuring cells are limited by the strength of their construction and are not appropriate for measurements at either high pressures or high mechanical stresses.
Flow measuring cells are often used in high performance liquid chromatography (HPLC) analysis and capillary electrophoresis. Flow cells of this type are typically constructed of strong/durable materials, have small volumes, and generally have a short sample “path length”. Additional requirements are imposed for industrial process applications due to the elevated mechanical stresses caused by high pressures and high process flow rates. These cells must also be designed to resist the potentially damaging effects of the process medium, e.g., the inlet and outlet channels can be arranged at an angle to the light path direction to reduce mechanical stress on optical windows. The cell-holder may be made of an inert plastic to increase the cell life time when used with corrosive process streams.
Some flow cells are utilized for optical measurements at high process pressures. Other flow cells are designed for light absorption measurements of small liquid volumes. Nickel capillary tubes with polished interior surfaces have been utilized for optical measurements using multiple radiation reflection.
Flow cells with multiple capillary analyzers have been designed for biochemical capillary analysis. The use of flow cells of this type allows minimization of the sample volume. Short path length flow cells may be used for measurements obtained in the ultraviolet, visible or middle infrared region of electromagnetic radiation spectrum. However, the relative absorbance values observed in near infrared region of electromagnetic radiation spectrum are significantly less, thus requiring longer path lengths to minimize sample analysis errors.
Liquids may also contain dissolved gases at high pressure that can interfere with spectroscopic measurements. The arrangement for analytical control of metallurgical production can include an air-separating vessel installed before the measuring cell.
As described above, optical measurements have been carried out for a variety of different process conditions. However, the prior art does not describe a suitable cell design for use in the measurement of process streams containing one or more immiscible liquid phases, or for process streams containing dissolved or bulk gases. The previously described cells may be used for the analysis of homogeneous samples but are inadequate for measurements of process samples having immiscible phases. Process samples containing one or more immiscible phases result in significant light dispersion at the interface of the various phases and result in unacceptable measuring errors.
What is needed in the art is a flow cell capable of measurement of immiscible liquid phases.